The present invention relates generally to fabrication of integrated circuits, and more particularly, to method and apparatus for formation of an in-situ doped amorphous semiconductor film on semiconductor wafers with reduced defects and predictable electrical characteristics.
A doped amorphous semiconductor film is used in integrated circuits for various purposes such as for forming a gate structure of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The doped amorphous semiconductor film in the prior art is typically formed by first forming a semiconductor film that is substantially undoped and then implanting dopant into the semiconductor film to enhance the conductivity of the semiconductor film. The dopant that has been implanted into the semiconductor film is then activated in a thermal anneal process to further enhance the conductivity of the semiconductor film from activation of the dopant within the semiconductor film, as known to one of ordinary skill in the art of integrate circuit fabrication.
However, such an implantation process is disadvantageous because three steps are used for forming the doped semiconductor film including formation of the undoped semiconductor film, implantation of the dopant into the semiconductor film, and then a thermal anneal process for activation of the dopant within the semiconductor film. Each additional step during integrated circuit fabrication introduces added cost and delay to the manufacture of integrated circuits.
A chemical reaction process such as a chemical vapor deposition process is used for formation of a doped semiconductor film with in-situ doping of the semiconductor film. With xe2x80x9cin-situxe2x80x9d doping, dopant is incorporated into the semiconductor film simultaneously during deposition of the semiconductor film. Thus, a chemical reaction process for formation of the semiconductor film with xe2x80x9cin-situxe2x80x9d doping avoids the three steps of the dopant implantation process.
Nevertheless, a chemical reaction process of the prior art is disadvantageous because defects may tend to form more easily in such a chemical reaction process than in an implantation process. Furthermore, electrical characteristics such as the sheet resistance of the semiconductor film is typically harder to control in a chemical reaction process than in an implantation process. In addition, the uniformity of electrical characteristics such as the sheet resistance of the semiconductor film across the semiconductor wafer is also typically harder to control in a chemical reaction process than in an implantation process.
For avoiding the three steps of the implantation process, an improved chemical reaction process is desired for formation of a semiconductor film with in-situ doping of the semiconductor film and with reduced defects and with predictable electrical characteristics such as controllable and uniform sheet resistance across the semiconductor wafer.
Accordingly, in a general aspect of the present invention, an improved LPCVD (Low Pressure Chemical Vapor Deposition) process is used for formation of a doped amorphous semiconductor film with in-situ doping of the semiconductor film on a plurality of semiconductor wafers with reduced defects and with predictable electrical characteristics.
In one embodiment of the present invention, the plurality of semiconductor wafers are placed in a reaction chamber. The pressure within the reaction chamber is set to be less than approximately 1.0 Torr, and the temperature within the reaction chamber is set to a predetermined temperature in a range of from about 500xc2x0 Celsius to about 550xc2x0 Celsius. A semiconductor film reactant and a dopant reactant are introduced into the reaction chamber through at least two gas inlets. Each gas inlet is disposed on a respective location within the reaction chamber near the plurality of semiconductor wafers, and each gas inlet carries both of the semiconductor film reactant and the dopant reactant. A doped amorphous semiconductor film is formed from the semiconductor film reactant with in-situ doping from the dopant reactant on the plurality of semiconductor wafers in a LPCVD (Low Pressure Chemical Vapor Deposition) process.
In this manner, with the at least two gas inlets carrying the semiconductor film reactant and the dopant reactant in the LPCVD (Low Pressure Chemical Vapor Deposition) process, the doped amorphous semiconductor film deposited on the plurality of semiconductor wafers has more predictable and uniform electrical characteristics such as sheet resistance across a semiconductor wafer.
The present invention may be used to particular advantage when a doped amorphous silicon film is deposited with the semiconductor film reactant being comprised of silane (SiH4) and with the dopant reactant being comprised of phosphine (PH3) for in-situ doping of phosphorous in the doped amorphous silicon film.
Furthermore, for reducing defects on the semiconductor wafer during the LPCVD (Low Pressure Chemical Vapor Deposition) process, a vacuum suction may be applied through the at least two gas inlets and nitrogen (N2) gas may be purged through the at least two gas inlets alternatingly at least three times before and after deposition of the doped amorphous semiconductor film. In addition, a layer of undoped semiconductor film, such as undoped amorphous silicon film for example, is deposited on a boat that holds the plurality of semiconductor wafers within the reaction chamber when the boat is empty and on components of the reaction chamber before deposition of the doped amorphous semiconductor film.
These and other features and advantages of the present invention will be better understood by considering the following detailed description of the invention which is presented with the attached drawings.